performResize method

@override

voidperformResize
()

override

Updates the render objects size using only the constraints.

Do not call this function directly: call layout instead. This function
is called by layout when there is actually work to be done by this
render object during layout. The layout constraints provided by your
parent are available via the constraints getter.

Subclasses that set sizedByParent to true should override this method
to compute their size.